Existing cleaning methods of the germanium surface involve wet chemical etching, ion bombardment, or very high temperature processing. The latter two processes are incompatible with commercial semiconductor logic processing. The wet cleaning method is extremely difficult to integrate into common process modules. The wet cleaning methods roughen the Ge surface leaving a surface that is disordered and/or left with contaminants. The disorder can be reduced by high temperature processing (875° C.), which is undesirable in commercial semiconductor processing because it induces dopant diffusion.
There are numerous previous reports of wet H2O2 cleaning and high dose H cleaning of Si, SiGe, and Ge. The Ge alloys (SiGe and GeSb) are important since they will likely be employed in commercial devices prior to pure Ge for channels, sources, and drains. Anthony et al. described that cleaning of silicon surfaces by remote RF hydrogen plasma in mTorr range after wet cleaning can remove carbon and oxygen from the Si(100) surface. B. Anthony et al., “In situ Cleaning of Silicon Substrate Surfaces by Remote Plasma-Excited Hydrogen”, J. Vac. Sci. Technol. B 7(4), (1989). A modification with wet cleaning and dosing with a remote H plasma source at an optimized Si sample temperature of 250° C. demonstrated both carbon and oxygen could be removed from Si(100) to produce a flat surface and a sharp RHEED pattern. D. Kinosky et al., “Hydrogen Plasma Cleaning of the Si(100) Surface: Removal of Oxygen and Carbon and The Etching of Si,” Materials Research Society Proc 315, (1993). The need to avoid plasma damage is critical; Tae et al. developed a defect-free, in situ cleaning of wet cleaned silicon using an ECR UHV hydrogen plasma treatment at a 560° C. surface temperature; however, to avoid ion damage a positive voltage had to be applied to the sample. See, Tae, et al., “Low-Temperature in situ Cleaning of Silicon (100) Surface by Electron Cyclotron Resonance Hydrogen Plasma,” J. Vac. Sci. Technol. B 13, 908 (1995).
For SiGe, similar results have been reported. Li et al. employed ECR atomic H cleaning with 20 eV ion energy after wet cleaning with HCl:H2O2 and HF to remove metallic and organic impurities from SiGe samples at 250° C., but it is unclear if oxygen was removed. Li et al., “SiGe Gate Oxide Prepared at Low Temperatures in an Electron Cyclotron Resonance Plasma,” Appl. Phys. Lett. 63, 2938 (1993).
The work described by Jones et al. is the only previous dry cleaning method known to the present inventors for a germanium containing semiconductor device. That process employed UV ozone and an extremely high temperature and long duration (>12 hr) anneal. See, D. E. Jones et al. “Scanning Tunneling Microscopy Study of Cleaning Procedures for SiGe(001) Surfaces,” Surf. Sci Vol. 341, No. 1, pp. 1005-1010 (1995). The completely gas phase process produced a 1-2 nm oxide layer, but required extensive annealing up to 1050° C. for oxide desorption while maintenance of doping profiles in semiconductor requires processing below 500 C.
Others have reported that atomic hydrogen can be an effective way to remove oxygen and possibly carbon contamination from the Si/SiGe surfaces during cleaning, and the H does serve as a cleaning mechanism agent. Either high atomic H cleaning doses or wet cleans were employed, both of which induce etching. Since there is a great interest in using Ge as channel material in FinFETs, any ex-situ acid wet cleaning or high atomic H dose procedure should be avoided to minimize etching and surface roughness. Prabhakarana, et al., “An efficient Method for Cleaning Ge(100) Surface,” Surface Science Volume 316, Issues 1-2, 1 Sep. 1994, Pages L1031-L1033.
A known passivation method uses H2O gas phase dosing to passivate and functionalize an already cleaned surface. See, J. S. Lee, et. al., “Atomic Imaging of Nucleation of Trimethylaluminum on Clean and H2O Functionalized Ge(100) surfaces,” Journal of Chemical Physics, 135, 054705, (2011). Other gas phase passivation methods include nitridation and oxidation. Nitridation of the Ge surface is typically performed using a plasma source to produce a thermally stable Ge oxynitride or Ge nitride layer in order to suppress the out-diffusion of GeO from the Ge surface into the high-k dielectric layer during the post-deposition annealing process. Oxidation using ozone or high pressure O2 is also found to passivate the Ge surface by forming a stoichiometric GeO2 layer, which minimizes the suboxide species at the interface. However, to scale down the equivalent oxide thickness (EOT) of the Ge-channel MOSFET device, the thickness of these passivation layers has to be reduced to about one monolayer (ML).
Others have reported passivation methods on the SiGe(100) surface including growth of a Si passivation layer. These methods can result in non-uniform growth unless very thick passivation layers are used. Yang et. al. reported on the passivation of the SiGe surface using an Al2O3 layer but the Al2O3 layer was very thick (20 nm), which would be bad for scaling of the equivalent oxide thickness (EOT) in MOSFETs, where passivation layers more than 1 nm are not acceptable. See, Yang et al., “Effective passivation of defects in Ge-rich SiGe-on-insulator substrates by Al2O3 deposition and subsequent post-annealing,” Solid State Electronics, Volume 60, Issue 1, pp 128-133. The Al2O3 passivation resulted in SiO2 film growth.
Thermal oxidation passivation methods of the SiGe surface lead to preferential oxidation of Si species. This produces Ge-rich layers near the SiGe-oxide interface, which is known to cause degradation of the oxide properties.